Update CONTRIBUTING.md (#289)

Author: graysonchen

CONTRIBUTING.md

@@ -53,6 +53,8 @@ The git hooks handle style, tests, and file generation for you. No excuses for b
 
 ## Testing
 
+Run linting: `bundle exec rubocop`
+
 Run tests: `bundle exec rspec`
 
 **Re-recording VCR cassettes?** Set API keys and run:
@@ -80,4 +82,4 @@ Consider [sponsoring RubyLLM](https://github.com/sponsors/crmne) to help with on
 
 That's it. Let's make Ruby the best AI development experience possible.
 
-— Carmine
+— Carmine